Sunnhemp as a green manure
Based on EDN issues 26 and 36. Fr. Gerold Rupper in Tanzania reports that sunn hemp (Crotalaria ochroleuca) is receiving widespread acceptance as a versatile green manure in East Africa. The jack bean, velvet bean and lablab bean are all vines. Sunn hemp is a vigorous upright legume growing 2 meters tall. When planted in narrow rows, mature plants tend to fall over. When planted in the field plants tend to hold each other up. While sunn hemp has a different growth habit than most of the green manures we have featured, the uses are much the same, including: weed control, livestock feed, and erosion control. Sunn hemp is especially suited for weed control in fruit groves because, unlike vining ground covers, continual vigilance to keep it from covering the trees is not necessary. It is being used with banana, plantain, citrus, and coconut. It can be cut at any time and left in the field as mulch. If it is cut one foot (30 cm) from the ground it will grow a second time. Fr. Ruber stresses that not less than 10 kilo of seed per acre must be planted.
Fr. Rupper wrote, "In Hanendi, sunn hemp was planted in an orchard affected badly by insects. When it had grown a bit, the insects left the trees and started to live on the sunn hemp. When the sunn hemp was cut for mulching, the insects returned to the orange trees." "Just this week we were informed that insects which attacked the freshly planted maize moved to inter-cropped sunn hemp, ate the roots and are perishing." Crotalaria is known to contain toxins, but this variety is free of toxin, except perhaps the seed. It is cut about 3 months after planting. It is best cut in the morning, but keeps until evening. Later in the season cattle can be allowed to graze in the sunn hemp field. One farmer noted that after first spending an hour in a grass field, his cows even ate the dry stems.
Fr. Ruper mentions that cattle must not be allowed to spend more than about one hour in the area. [He does not say why.] He also says that the seeds should not be stored in a closed room where people are working. Sunn hemp seeds are used to keep weevils from stored rice and maize. Sunn hemp seeds are spread over the ground and bags put on top of the seeds. This procedure is continued, layering sunn hemp seed and bags of stored grain. After about 9 months, the process must be repeated.
When we asked our EDN readers for suggestions on how to keep monkeys out of the garden, Fr. Rupper wrote: "Early in the campaign for planting sunn hemp (also called zanziberica), we got a report from a youth group that monkeys had been afraid to traverse a belt of sunn hemp around their field of maize. I could not ask the monkeys why they did so. But one can imagine that first of all it is a strange sight to see sunn hemp growing together and forming a barrier.
Secondly, the husks give a clattering sound, which may disturb the monkeys. [Editor: The genus for rattle snake is crotalus coming from the Greek root crotal meaning a rattle or castanet].
Thirdly, if they are caught stealing maize, it is almost impossible to flee through the sunn hemp field as the branches form a rather strong network like wire. In the case of maize [corn] there is some synchronization between the crop and sunn hemp. The husks of both crops form about the same time (depending on the variety of maize). People like to let the corn dry in the fields, at which time the barrier effect of sunn hemp becomes important....
Meanwhile we have developed a new method of planting sunn hemp. Two rows of maize alternate with one row of sunn hemp. Here the maize is well protected against monkeys." As with velvet bean, farmers are especially appreciative of its usefulness in controlling weeds and improving the texture of the soil. He tells farmers, "If you have no chemical fertilizer when the season starts, plant sunn hemp between your food crops. If fertilizer arrives you may still be able to use it. If not, use sunn hemp and you will at least get a modest crop." According to Fr. Rupper sunn hemp will completely kill striga. A simple alley cropping system has been developed for controlling this important weed. When a field is ploughed and sowed to corn or sorghum, sunn hemp is sown along with the grain at a rate of 10 kilos (mixed with 20 kilos of sand) per acre. At weeding time, sunn hemp is left standing in every third row, knowing that it will kill the crop.
After seven or eight months sunn hemp seeds are harvested and the dry stems are placed in the furrows and buried. If this is practiced each year you have a sustainable system free of striga. Other uses for sunn hemp include: applying the dry stems and any husks to trees or gardens as mulch, or as bedding for livestock. The seeds, about the size of millet, are mixed with two parts of coarse sand and broadcast by hand. They do not need to be covered, although it might be well to draw a branch across the newly planted field. They sprout after a few days and develop a strong root. Growth is rather slow until they reach about one foot, then they quickly grow to 2 meters or more. Sunn hemp is fairly drought resistant, recovering well when rains return. Plants bare seed after 3-4 months and die after 6 months. However, if they are cut back to about one foot (30 cm) above the ground, they again develop new leaves. If planted densely in a well-prepared field, no further work is needed (except to keep out animals).
Sometimes sunn hemp is interplanted with maize. Some species of Crotalaria are also useful in suppressing nematodes, but we do not know if this is one of them. ECHO also carries another species of sunn hemp, Crotalaria juncea.
Variety 'Tropic Sun', released by the University of Hawaii, is included in rotation with vegetables, ornamentals and others to add nitrogen, organic matter, suppress weeds, control erosion and reduce root-knot nematodes. In 60 days it can produce 145 pounds of nitrogen and 3 tons of dry matter per acre. Seed should be broadcast at the rate of 40-60 pounds per acre and covered 1/2 inch deep. High populations make the stems more succulent and hence better for incorporation into the soil. If allowed to grow too tall, stems become fibrous and difficult to deal with. Seeds can be inoculated with cowpea inoculant to maximize nitrogen fixation [presumably not needed where cowpeas are commonly grown]. It also lacks the poisonous alkaloids that make some species of crotalaria poisonous to livestock.
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